High pressure metal vapor lamps generally comprise an inner envelope or arc tube which is enclosed within a vitreous outer envelope or jacket. In metal halide lamps, the arc tube is of quartz or fused silica and contains a quantity of mercury along with one or more metallic halides and an inert starting gas. In commercial manufacture, the arc tubes have been commonly made utilizing so-called full pinch seals wherein the entire end segments of a piece of quartz tubing are flattened and sealed off. It is done by pressing the ends of the tube while in a heat-softened condition between a pair of opposed jaws to collapse the quartz about a foliated inlead supporting an electrode on its inner end. This results in a fin-like configuration resembling somewhat a fish's tail at each end of the arc tube.
When a metal halide lamp has reached a stable operating temperature, the mercury in the fill is substantially all vaporized but an excess of metal halide remains unvaporized. Such excess collects at cold spots within the arc chamber and these tend to be the ends or corners of the curved seam produced by the pinch. Such non-uniform condensation of the excess metal halide in flat-pinched arc tubes is often referred to as the fin-effect.
In U.S. Pat. No. 4,161,672--Cap and Lake, July 1979, the advantages of small inlead seals which have a low radiation-blocking or absorptive cross section are explained. They are particularly desirable in low wattage or miniature arc lamps in order to achieve high efficacy. One way of making a seal with a low absorptive cross section is by heating to a viscous state the seal portion of a preformed bulb, and vacuum-collapsing it onto the foil portion of an electrode inlead extending through it. Since in practice it is necessary to revolve the bulb as the neck is being heated and collapsed, the equipment required to seal arc tubes in this way is more elaborate and expensive than that required for pinch-sealing.
The object of this invention is to provide an arc tube configuration or design which achieves a low absorptive cross section and avoids fin-effect and which can be achieved through the use of pinching jaws. An ancillary object is to provide a convenient and economical method of manufacture for arc tubes of such configuration.